<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .a{
            width: 0;
            height: 0;
            border-left: 20px solid transparent;
            border-right: 20px solid transparent;
            border-bottom: 30px solid #000000;
        }

    </style>
</head>
<body>
    <div class="a"></div>
</body>
</html>